There is a constantly growing demand for electrical power. Most electricity generated today is produced by magnetic field interaction with a conductor. Rotary generators—such as power station generators, car alternators, dynamos, and the like—move a magnetic field and a conductor relative to each other, and generate electricity. However this type of machinery requires kinetic energy, largely derived from fossil fuel, which is expensive and carries an environmental cost.
In solar power systems, energy is obtained by directly harnessing the electromagnetic solar radiation from the sun. The total solar radiation incident on the earth far exceeds the total power requirements well into the future and offers a viable offset to fossil fuels. Significant effort has been directed to higher utilization of solar energy but efficiency, space, and cost considerations hampered this effort to date.
In its most basic form, the term ‘refraction’ means the change of direction of a ray of light, sound, heat, radio waves, and other forms of wave energy, as it passes from one medium to another. Generally waves of different frequencies would refract at different angles and thus refraction tends to spatially separate multispectral radiation into its component frequencies.
Electromagnetic (EM) radiant energy extends over a broad frequency spectrum, however many applications deal only with portions of this spectrum. Light is one form of radiant energy which may be considered as an alternating EM radiation at very high frequency. Humans perceive different light frequencies as different colors, and there is a large amount of radiation that is not perceived by humans, generally known as UV (Ultra Violet), and IR (Infra Red), and the term light will be extended thereto. Visible light ranges generally between 760-300 nm and roughly corresponds to the peak intensity of solar radiation transmitted through the atmosphere. Infrared radiation ranges from the extreme far end of 1 mm (33 THz; millimeter radio waves) to about 760 nm. As the human eye is capable of directly sensing and differentiating between light of different frequencies, it will be used oftentimes to explain the operation of different aspects of the invention for the sake of brevity and increased clarity. However the spectral range of interest of different embodiments of the present invention, and the principle described herein, extend to any electromagnetic radiant energy and thus the all or portions of the solar and infrared (IR) spectrum unless otherwise specified or clear from the context.
For solar energy applications the spectral range of interest will likely be a spectrum containing most if not all of the solar spectrum available at the location where the solar cell is to be deployed, or the portion thereof which is economically used by the device at hand, typically of wavelength within the 1.8 μm to 300 nm. Yet, for devices directed to heat energy recovery, it is likely that only the infra-red portion of the spectral range is of interest. Similarly, the spectral range of interest may be applicable to portions of a device, such that by way of example, a device may be directed to a broad spectrum, but portions thereof may be directed to a narrower spectrum, and the spectral range of interest is thus limited to the range of interest for that portion of the device. By way of example a television may occupy a display portion, and additional emissions such as audio outputs. The spectral range of interest of the display may only extend to the visible range, even if the device as a whole includes the aural range as well, the aural range does not fall within the spectral range of the display used in the television. It is seen therefore that the application at hand determines the spectral range of interest, and that a spectral range of interest may differ by application, an apparatus, or a portion thereof. Regarding lateral waveguides, yet another aspect described below, each waveguide may have its own spectrum of interest, which may differ from the spectral range of interest of an adjacent waveguide.
Therefore, the spectral range of interest, equivalently referred to as the spectrum of interest, is defined herein as relating to any portion or portions of the total available spectrum of frequencies which is being utilized by the application, apparatus, and/or portion thereof, at hand, and which is desired to be detected and/or emitted utilizing the technologies, apparatuses, and/or methods of the invention(s) described herein, or their equivalents.
The term spectral component will relate to a portion of the energy at the aperture, which is characterized by its frequency, polarization, phase, flux, intensity, incidence, radiosity, energy density, radiance, or a combination thereof.
Waveguides are a known structure for trapping and guiding electromagnetic energy along a predetermined path. An efficient waveguide may be formed by locating a layer of dielectric or semiconducting material between cladding layers on opposite sides thereof. The cladding may comprise dielectric material or conductors, commonly metal. Waveguides have a cutoff frequency, which is dictated by the wave propagation velocity in the waveguide materials, and the waveguide width. As the frequency of the energy propagating in the waveguide approaches the cutoff frequency Fc, the energy propagation speed along the waveguide is slowed down. The energy propagation of a wave along a waveguide may be considered as having an angle relative to cladding. This angle is determined by the relationship between the wavelength of the wave and the waveguide width in the dimension in which the wave is being guided. If the width of the waveguide equals one half of the wave wavelength, the wave reaches resonance, and the energy propagation along the waveguide propagation axis stops. Two dimensional and three dimensional waveguides are well understood and their behavior is commonly modeled using mathematical models.
A Continuous Resonant Trap Refractor (CRTR) is a novel structure which is utilized in many aspects of the present invention, and as such, a simple explanation of the principles behind its operation is appropriate at this early stage in these specifications, while further features are disclosed below. A CRTR is a structure based on a waveguide having a tapered core, the core having a wide base face forming an aperture, and a narrower tip. The core is surrounded at least partially by a cladding which is transmissive of radiant energy under certain conditions. The CRTR may be operated in splitter mode and in a mixer/combiner mode. In splitter mode the radiant energy wave is admitted into the CRTR via the aperture, and travels along the depth direction extending between the aperture and the tip. The depth increases from the aperture towards the tip, such that larger depth implies greater distance from the aperture. Due to the core taper, when multi-frequency radiant energy is admitted through the CRTR aperture, lower frequency waves will reach cladding penetration state before higher frequency waves, and will penetrate the cladding and exit the waveguide at a shallower depth than at least one higher frequency wave. As waves of differing frequencies will be emitted via the cladding at differing depths, the CRTR will provide spatially separated spectrum along its cladding. Conversely, when operated in mixer/combiner mode, a wave coupled to the core via the cladding, at or slightly above a depth where it would have reached CPS in splitter mode, will travel from the emission depth towards the aperture, and different frequencies coupled through the cladding will be mixed and emitted through the aperture. Coupling light into the CRTR core from the cladding, will be related as ‘injecting’ or ‘inserting’ energy into the CRTR.
The depth at which the wave would couple into the tapered core is somewhat imprecise, as at the exact depth of CPS the wave may not couple best into the core, thus the term ‘slightly above’ as referred to the coupling of light into the tapered core in combiner/mixer mode should be construed as the depth at which energy injected into tapered core via the cladding would best couple thereto to be emitted via the aperture, within certain tolerances stemming from manufacture considerations, precision, engineering choices and the like.
Tapered waveguide directed at trapping radiant energy, as opposed to emitting energy via the cladding, have been disclosed by Min Seok Jang and Harry Atwater in “Plasmionic Rainbow Trapping Structures for Light localization and Spectrum Splitting” (Physical Review Letters, RPL 107, 207401 (2011), 11 Nov. 2011, American Physical Society©). The article “Visible-band dispersion by a tapered air-core Bragg waveguide”, (B. Drobot, A. Melnyk, M. Zhang, T. W. Allen, and R. G. DeCorby, 8 Oct. 2012/Vol. 20, No. 21/OPTICS EXPRESS 23906, ©2012 Optical Society of America “Visible-band dispersion by a tapered air-core Bragg waveguide” B. Drobot, A. Melnyk, M. Zhang, T. W. Allen, and R. G. DeCorby, 8 Oct. 2012/Vol. 20, No. 21/OPTICS EXPRESS 23906, ©2012 Optical Society of America) describes out-coupling of visible band light from a tapered hollow waveguide with TiO2/SiO2 Bragg mirrors. The mirrors exhibit an omnidirectional band for TE-polarized modes in the ˜490 to 570 nm wavelength range, resulting in near-vertical radiation at mode cutoff positions. Since cutoff is wavelength-dependent, white light is spatially dispersed by the taper. These tapers can potentially form the basis for compact micro-spectrometers in lab-on-a-chip and optofluidic micro-systems. Notably, Bragg mirrors are very frequency selective, complex to manufacture, and require at least a width higher than ¾ wavelength to provide any breadth of spectrum. In addition to the very narrow band, the Bragg mirrors dictate a narrow bandwidth with specific polarization, while providing however a fine spectral resolution.
UV radiation ranges between the visible light and higher frequency electromagnetic energy, such as X-Rays and gamma rays. While large amounts of UV energy are absorbed in the atmosphere, this radiation may still be of interest for harvesting above about 300 nm. For many applications and embodiments, the spectral range of interest may occupy only a portion of the available spectrum.
At sufficiently high frequencies, radiant energy is also commonly considered as a flow of photons, which are quantized units of energy which increases with frequency. Under this quantum physics description, the energy density associated with electric and magnetic fields are probability distributions of photons. Therefore certain terms that are common to simple electromagnetic energy can be better clarified as relating to the spectrum of interest. Thus, a dielectric material in the above mentioned energy spectrum of interest relates to a material having low conductivity, and having a band-gap between a filled valence band and an empty conduction band exceeding the energy of any photon in the spectrum of interest to a specific application. A “semiconductor” refers to a photovoltaicly active material, having a bandgap comparable to or smaller than the photon energy of any photon in the spectrum of interest to a specific application.
In contrast, a transparent conductor is a material having a finite but meaningful conductivity due to a partially filled conduction band or partially empty valence band but having a band-gap between the valence band and conduction band exceeding the energy of any photon in the spectrum of interest. These materials act like a dielectric at high frequencies but act like a conductor at low frequencies. Transparent dielectric materials also have low optical losses such that photons efficiently transmit through such material, at least at the spectrum of interest or a significant portion thereof.
While transparent conductors may be considered as wide bandgap semiconducting materials, they are used as conductors in most applications. Dielectrics, transparent conductors, and semiconductors, as used in these specifications, refer to materials that have a dielectric constant at optical frequencies; however the distinction between a semiconductor and the remaining materials is that the bandgap of a semiconductor is not substantially larger than the photon energy. As a general and non-limiting guideline, table 1 describes several characteristics of the different conductive, insulating, and semi-conductive materials.
TABLE ITramsparentSemi-MaterialMetalConductorconductorDielectricBandgap→ 0>>photon≦photon>>photonDC ConductivityhighgoodVaries→ 0Optical PropertyreflectivetransparentabsorptivetransparentDielectric constantcomplexlow losslossylow loss
In these specifications, the term cladding penetration state relates to a condition where energy confined by a tapered core waveguide leaves the waveguide via the cladding. Generally each waveguide has some negligible penetration of energy of energy into the cladding, however cladding penetration state occurs when a significant amount of energy is transported through the cladding. Cladding penetration state is generally frequency related, and energy of one frequency may reach cladding penetration state at a different set of conditions than the cladding penetration state of another frequency. By way of non-limiting example, if 66% of the energy of frequencies between F1 and F2 will exit a hypothetical waveguide via the cladding at a distance between 1 um to 2 um from the waveguide aperture, the cladding penetration state for F1-F2 would exist between 1-2 um from the aperture. Other frequencies may or may not overlap such range partially or completely. Notably the number 66% has been arbitrarily selected by way of example only, and may be modified as an engineering choice according to the application at hand.
In these specifications, cladding penetration state is used primarily to define a location or a region where cladding penetration would occur, rather than necessarily the actual occurrence of cladding penetration.
Stationary resonance condition is a condition in a waveguide where the local cutoff frequency of the waveguide equals the frequency of a wave guided by the waveguide, such that the guided wave reflects repeatedly between opposing surfaces of the guide, however the corresponding component of energy velocity along the waveguide propagation axis is zero. As the wave frequency approaches the local cutoff frequency of the waveguide, a sharp decrease in the wave propagation (group) velocity is noticed at the immediate vicinity of the cutoff dimension, as may be seen by way of example in the lower graph of FIG. 3. While complete stationary resonance condition is seldom if ever achievable, for the purpose of these specifications a stationary resonance (SRC) condition will be considered a situation where the guided wave is sufficiently close to the complete stationary resonance condition to significantly lower than the speed of light in the bulk material of the waveguide. Stated differently, when a wave falls within the zone of the sharp decrease in velocity it is considered to be in SRC.
With proper selection of cladding material and dimensions, energy will reach a cladding penetration state and depart the waveguide through the cladding at this stationary resonant condition. This mechanism is related to by the acronym CPS-SRC. CPS-SRC often occurs with reflective cladding, comprising thin metallic cladding. Notably a metallic cladding of lower thickness than the penetration depth to which the cladding is locally exposed would allow energy to pass therethrough and such cladding may be utilized. Furthermore, when certain metals are disposed at low thicknesses they tend to “ball-up” and form small “islands”. Such “balled-up” metal, and/or intentionally perforated metal cladding may also form a discontinuous metal film cladding in a reflective CRTR waveguide.
Total internal reflection (TIR) is a phenomenon which occurs when a guided wave hits the boundary between the core and the cladding below a certain angle relative to the local propagation axis of the waveguide. The angle is known as the critical angel of total Internal Reflection. When a guided wave reaches or exceeds the critical angle it departs the waveguide via the cladding under normal refraction. Slightly below this critical angle the internal reflection by a finite cladding becomes incomplete in a process known as Frustrated Total internal Reflection (FTIR). This condition occurs mostly with dielectric cladding, but metallic claddings with small perforations or with thicknesses at or near the tunnel distance also have angle dependent reflection coefficients, resulting in a situation analogous to FTIR. Cladding penetration condition reached by a wave exceeding the critical angle of total internal reflection is referred to hereinafter as CPS-FTIR. Both CPS-FTIR and CPS-SRC are characterized by energy traversing the cladding, thus CPS, or ‘cladding penetration state’ will be used interchangeably to denote CPS occurring through any mechanism.
Structure to facilitate conversion of radiant energy to electricity or electrical signals (hereinafter “LE”), or conversion of electrical signals into radiant energy such as light (hereinafter “EL”) are known. Collectively, objects, materials, and structures, which perform conversion between two forms of energy, or adjust and control flow of energy, are known by various names which denote equivalent structures, such as converters, transducers, absorbers, detectors, sensors, and the like. To increase clarity, such structures will be referred to hereinunder as ‘transducers’. By way of non-limiting examples, the term “transducer” relates to light sources, light emitters, light modulators, light reflectors, laser sources, light sensors, photovoltaic materials including organic and inorganic transducers, quantum dots, CCD and CMOS structures, LEDs, OLEDs, LCDs, receiving and/or transmitting antennas and/or rectennas, phototransistors photodiodes, diodes, electroluminescent devices, fluorescent devices, gas discharge devices, electrochemical transducers, and the like.
A transducer of special construction is the RL transducer, which is a reflective transducer. Reflective transducers controllably reflect radiant energy. Such transducers may comprise micro-mirrors, light gates, LCD, and the like, positioned to selectively block the passage of radiant energy, and reflect it into a predetermined path, which is often but not always, the general direction the energy arrived from. Certain arrangements of semiconductor and magnetic arrangements may act as RL transducers by virtue of imparting changes in propagation direction of the radiant energy, and thus magnetic forces or electrical fields may bend a radiant frequency beam to the point that in effect, it may be considered as reflected. RL transducers may be fixed, or may be used to modulate the energy direction over time.
The skilled in the art would recognize that certain LE transducers may act as EL transducers, and vice versa, with proper material selection, so a single transducer may operate both as EL and LE transducer, depending on the manner of operation. Even certain RL transducers may act as another transducer type. Alternatively transducers may be built to operate only as LE, as RL, or as EL transducers. Furthermore, different types of transducers may be employed in any desired combination, so the term transducers may imply any combinations of LE, EL, and RL, as required by the application at hand.
Solar energy converters have been extensively researched and many attempts were made to achieve an efficient structure for performing solar energy to electric energy conversion. Presently the most common devices for achieving such conversion are photovoltaic (PV) solar cells which generally use layers of different materials forming a PN junction at their interface. When exposed to a photon having energy equaling or higher than the band gap between the junction materials, the photon energy causes formation of electron-hole groups, which are separated and collected on both sides of the junction. Several technologies exist to increase efficiency in the PV transducer, such as multi-layered junctions, quantum dots, and the like.
Yet another class of transducers employs polymer based photoabsorptive material as electron donors in combination with electron acceptors. In some cases the resulting excited electron is separated from the corresponding hole using different work function conductors. In other cases, a polymer electron acceptor forms a heterojunctions with the electron donor. Such transducers generally have lower efficiency; however their efficiency may be significantly advanced by aspects of the present invention.
Other types of transducers utilize antennas, and more commonly rectennas, to achieve the energy conversion. The term rectenna relates to an antenna structure having a rectifier integrated with, or closely coupled to, the antenna, such that electromagnetic energy incident on the antenna is rectified and presented as primarily unidirectional (ideally DC) signal. By way of example, rectennas are described in U.S. Pat. No. 7,799,998 to Cutler, and in “Nanoscale Devices for Rectification of High Frequency Radiation from the Infrared through the Visible: A New Approach”, N. M. Miskovsky, P. H. Cutler, A. Mayer, B. L. Weiss, Brian Willis, T. E. Sullivan, and P. B. Lerner, Journal of Nanotechnology, Volume 2012, Article ID 512379, doi:10.1155/2012/512379, Hindawi Publishing Corporation©.
Current radiant energy detectors typically employ normal incidence of radiant electromagnetic energy onto a detector structure. Normal incidence has the limitation of a finite probability of detecting energy before it is transmitted through the collection layer. Energy transmitted through the collection layer is, at best, lost and, at worst, converted to heat in the supporting substrate. Several attempts has been made to provide transducers that use ‘side illumination’ in which the energy is inserted from the side of the junction. Such examples include, inter-alia, U.S. Pat. No. 3,422,527 to Gault, U.S. Pat. No. 3,433,677 to Robinson, and U.S. Pat. No. 4,332,973 to Sater.
Radiant energy transducers also typically employ a broad-band collector. Photovoltaic transducers are high pass collectors in that all energy above a critical cutoff frequency is converted. However, the photon energy in excess of the band gap energy is converted to heat. Rectenna transducers attempt to employ broad-band antennas with rectifiers. Their operating frequency is limited by the characteristics of the rectifier and by the bandwidth of the antenna. However, transducers generally exhibit a frequency dependent optimum efficiency, which is commonly also affected by temperature.
Stacking transducer layers of the same bandgap or of differing bandgap has been attempted and has been shown to improve efficiency. Detecting higher frequency signals in a first, higher bandgap material and transmitting lower frequency waves with photon energy below the material bandgap allows their subsequent conversion in lower bandgap materials. However, as each layer is of finite thickness, the transmitted signal losses are significant. Quantum dots are also frequency sensitive, and the technology offers a relatively easy tuning of the transducer to a specific band, and an ability to combine several frequency optimums with relative ease.
Radiant energy transducers typically employ normal incidence of radiant electromagnetic energy onto a detection structure. Normal incidence has the limitation of a finite probability of detecting energy before it is transmitted through the collection layer. Energy transmitted through the collection layer is, at best, lost and, at worst, converted to heat in the supporting substrate.
Prisms and other refractive devices can be used to improve incidence angles, and to direct different frequencies of radiant energy to different regions of a detector, where each region is optimized for a target frequency. U.S. Pat. No. 7,888,589 to Mastromattteo and U.S. Pat. No. 8,188,366 to Hecht, disclose examples of such devices. Different arrangements of concentrators are also known, which are operative to concentrate energy to transducers. U.S. Pat. No. 5,578,140 to Yogev et al. as well as Hecht provide examples to such arrangements. Those methods require significantly increased device area, and reduce the total energy per unit area (and per unit manufacturing cost) in exchange for increased efficiency.
Vertical optical waveguides are known in the art. U.S. Pat. No. 4,251,679 to Zwan depicts a plurality of transducing cavities having an inwardly inclined wall to receive impinging radiation. Two potential barrier strips each having different conduction electron densities; each potential barrier strip is connected to a conductor having a preselected conduction electron density whereby radiation impinging on a cavity will induce current flow which will be rectified across the potential barriers. U.S. Pat. No. 3,310,439 to Seney relates to embedding spaced dimensioned crystals into p-n semiconductor layers of a solar cell device. The crystals function as waveguides into the photovoltaic layer.
Waveguides, as well as different combinations of concentrators, mirrors, lenses, and other refractive devices have been used in different combination to increase the utility of various types of radiant energy transducers. However the existing solutions are either expensive, heavy or utilize large surface area relative to the usable transducer area. Furthermore, the above attempts are designed to have the radiant energy ideally impinges on the transducer at normal incidence angle for optimal efficiency.
The patents, patent applications, articles, and other publications disclosed above are incorporated herein by reference in their entirety.
There is therefore a clear, and heretofore unanswered, need for better technology, devices, and methods of manufacturer of energy conversion cells, which will solve the shortcomings of the known art, both for displays, and for energy conversion devices.